I don't think the booking class needs to be the same to do a SDC. How many W and lower fare class seats are available 12 hours from a flight? Would those people never be able to take advantage of SDC? And if the fare class has to match, why was I offered (through OLCI) an earlier flight that was not in the same class that I booked in?
It is there. While you usually can't buy a W or low booking class ticket 12 hours out (the fare rules on the cheaper tickets usually require 7/14/21-day advance purchase), there very often is availability at that bucket level.
